netdev - Re: [PATCH net-next v4 1/2] driver core: auxiliary bus: show auxiliary device IRQs

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <ae6e151e-0c34-4ff8-a9f7-40e4cbdb9dee@intel.com>
Date: Fri, 10 May 2024 16:01:01 +0200
From: Przemek Kitszel <przemyslaw.kitszel@...el.com>
To: Greg KH <gregkh@...uxfoundation.org>, Shay Drory <shayd@...dia.com>
CC: <netdev@...r.kernel.org>, <pabeni@...hat.com>, <davem@...emloft.net>,
	<kuba@...nel.org>, <edumazet@...gle.com>, <david.m.ertman@...el.com>,
	<rafael@...nel.org>, <ira.weiny@...el.com>, <linux-rdma@...r.kernel.org>,
	<leon@...nel.org>, <tariqt@...dia.com>, Parav Pandit <parav@...dia.com>
Subject: Re: [PATCH net-next v4 1/2] driver core: auxiliary bus: show
 auxiliary device IRQs

On 5/10/24 15:07, Greg KH wrote:
> On Fri, May 10, 2024 at 02:54:49PM +0200, Przemek Kitszel wrote:
>>>> +static ssize_t auxiliary_irq_mode_show(struct device *dev,
>>>> +				       struct device_attribute *attr, char *buf)
>>>> +{
>>>> +	struct auxiliary_irq_info *info =
>>>> +		container_of(attr, struct auxiliary_irq_info, sysfs_attr);
>>>> +
>>>> +	if (refcount_read(xa_load(&irqs, info->irq)) > 1)
>>>
>>> refcount combined with xa?  That feels wrong, why is refcount used for
>>> this at all?
>>
>> Not long ago I commented on similar usage for ice driver,
>> ~"since you are locking anyway this could be a plain counter",
>> and author replied
>> ~"additional semantics (like saturation) of refcount make me feel warm
>> and fuzzy" (sorry if misquoting too much).
>> That convinced me back then, so I kept quiet about that here.
> 
> But why is this being incremented / decremented at all?  What is that
> for?

[global]
This is just a counter, it is used to tell if given IRQ is shared or
exclusive. Hence there is a global xarray for that.
And my argument is for this case precisely.

[other]
There is also a separate xarray for each auxdev (IIRC) which is used as
generic dynamic container [that stores sysfs attrs], any other would
work (with different characteristics), but I see no problems with
picking xarray here.

> 
>> The "use least powerful option" rule of thumb is perhaps more important.
> 
> Yes, but use a refcount properly if needed, I can't figure out why a
> refcount is needed here at all, which is not a good sign.
> 
>>>> +	refcount_set(new_ref, 1);
>>>> +	ref = __xa_cmpxchg(&irqs, irq, NULL, new_ref, GFP_KERNEL);
>>>> +	if (ref) {
>>>> +		kfree(new_ref);
>>>> +		if (xa_is_err(ref)) {
>>>> +			ret = xa_err(ref);
>>>> +			goto out;
>>>> +		}
>>>> +
>>>> +		/* Another thread beat us to creating the enrtry. */
>>>> +		refcount_inc(ref);
>>>
>>> How can that happen?  Why not just use a normal simple lock for all of
>>> this so you don't have to mess with refcounts at all?  This is not
>>> performance-relevent code at all, but yet with a refcount you cause
>>> almost the same issues that a normal lock would have, plus the increased
>>> complexity of all of the surrounding code (like this, and the crazy
>>> __xa_cmpxchg() call)
>>>
>>> Make this simple please.
>>
>> I find current API of xarray not ideal for this use case, and would like
>> to fix it, but let me write a proper RFC to don't derail (or slow down)
>> this series.
> 
> Why do you need to use an xarray here at all?  Why isn't this just tied
> directly to the aux device instead?

for [global] above I find xarray suitable soultion, for the [other] I'll
leave defending it to @Shay :)

> 
> thanks,
> 
> greg k-h